1. Technical Field
The invention generally relates to amplifiers and, more particularly, to an interleaved amplifier employing interleaved signals for PWM ripple suppression.
2. Related Art
Pulse width modulation (PWM) amplification for audio applications has been used to increase efficiency by incorporating output devices that act as switches as opposed to linear devices that must dissipate a substantial amount of power. In PWM amplifiers, an audio input signal is converted to a pulse width modulated waveform. To this end, an audio signal is provided to the amplifier to modulate the width of an ultrasonic rectangular waveform based, for example, on the amplitude of the audio signal. The modulated waveform is used to drive one or more output devices as switches that are either fully saturated or off. The output devices, often implemented using switching power transistors, may be aligned in half-bridge pairs such that one device of the pair switches a positive voltage to the output, while the other device switches a negative voltage to the output. The switched output signals may be provided to the input of a low-pass filter in an attempt to remove harmonic signals and sidebands that are beyond the spectrum of the desired output waveform. The filtered analog signal is used to drive the load, such as a loudspeaker.
One set of pulse width modulated amplifier architectures, known as class-D amplifiers, are theoretically 100% efficient because the output transistors are either completely on, or completely off. These amplifiers, however, may be problematic since the timing of the switching of the transistors must be precisely controlled. In a class-D amplifier, the switches operate in time alternation. Ideally, the switching is perfectly timed so that one transistor instantaneously turns off as the other instantaneously turns on. If the switching is not perfectly timed, both the positive and negative switching devices may be on at the same time, allowing high “shoot-through” current, which may destroy the circuitry of a subsequent stage in the amplifier system. Therefore, in practice, a delay may be purposely introduced between the time at which one transistor turns off and the other transistor turns on. The time between the conduction intervals of the two switches when neither switch is on is known as deadtime. Deadtime may result in distortion and, therefore, should be minimized. Conversely, an insufficient amount of that time may result in undesired shoot-through current.
An amplifier addressing the shoot-through current and deadtime issues is available from Crown Audio International of Elkhart, Ind. The amplifier architectures used in certain of the Crown Audio amplifiers are known by various names including opposed current amplifiers, a balanced current amplifiers (BCA®), and “I-class” amplifiers. In this amplifier architecture, the positive and negative switching pulses corresponding to the modulated waveform are time interleaved with one another. When the audio input signal is at a zero-crossing, i.e. where no signal is to be provided at the amplifier output, the interleaved pulses turn the switches on and off in an overlapping manner at a 50% duty cycle. As a result the positive and negative power sources through-connected by the switches cancel each other out to provide a null output signal. When the incoming signal that is to be amplified exceeds the zero crossing and enters a positive voltage state, the duty cycle of the interleaved pulses are such that the duty cycle of the switch through-connecting the positive power source increases. When the incoming signal falls below the zero-crossing and goes to a negative state, the converse occurs.
Although the opposed current amplifier architecture provides a significant improvement over conventional PWM amplifiers, the architecture may be the subject of improvements. For example, as will be set forth in further detail below, the distortion results of an amplification system using multiple opposed current amplifiers that are interleaved with one another may be improved through the use of intelligently designed feedback systems.